1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Wireless communication systems typically include one or more base stations or access points for providing wireless connectivity to mobile units in a geographic area (or cell) associated with each base station or access point. Mobile units and base stations communicate by transmitting modulated radiofrequency signals over a wireless communication link, or air interface. The air interface includes downlink (or forward link) channels for transmitting information from the base station to the mobile unit and uplink (or reverse link) channels for transmitting information from the mobile unit to the base station. The uplink and downlink channels are typically divided into data channels, random access channels, broadcast channels, paging channels, control channels, and the like.
Mobile units can initiate communication with the base station by transmitting a message on one or more of the random access channels (RACHs). Uplink random access messages are non-synchronized and therefore may be transmitted at any time based on the synchronized downlink timing by any mobile unit within the coverage area of the base station. The receiver in the base station must therefore continuously monitor the random access channels and search the signals received on the random access channels for predetermined sequences of symbols (sometimes referred to as the RACH preamble) in random access messages transmitted by mobile units. To make the search process feasible, the format of the random access messages must be standardized. For example, conventional random access messages in the Universal Mobile Telecommunication Sservices (UMTS) Long Term Evolution (LTE) system are transmitted in a subframe during a transmission time interval (TTI) of 1 ms in 1.08 MHz bandwidth. The random access messages subframe is divided into a 0.8 ms preamble and a 102.6 μs cyclic prefix that includes a copy of a portion of the sequence of symbols in the preamble. The remaining 97.4 μs in the transmission time interval is reserved as a guard time to reduce or prevent inter-symbol interference between different random access messages.
The coverage area of a base station is related to the duration of the cyclic prefix and the guard time. For example, the conventional a guard time of approximately 0.1 ms corresponds to a round-trip delay for a signal that travels approximately 15 kilometers. Thus, a random access channel message format that includes approximately 0.1 ms for the guard time is appropriate for reducing or preventing inter-symbol interference for coverage areas or cell sizes having a radius of up to approximately 15 kilometers. Similarly, the duration of the cyclic prefix is related to the size of the coverage area. For example, a cyclic prefix of approximately 0.1 ms is suitable for coverage areas having radii of up to approximately 15 kilometers. Although a range of 15 km may be considered sufficient for conventional wireless communication systems, the base station range of proposed wireless communications systems, such as the UMTS LTE, is expected to increase to at least 100 km. Proposals to extend the range of the random access channel supported by base stations include increasing the transmission time interval to 2 ms.
FIG. 1 shows a first proposed modification to a random access message 100. In this proposal, the extended transmission time interval includes a 0.8 ms RACH preamble 105. The length of the cyclic prefix (CP) 110 increases in proportion to the desired coverage area. For example, every 0.1 ms of additional cyclic prefix length will account for additional 15 km coverage. The guard time 115 also increases at the same rate as the cyclic prefix length. Thus, with the 0.8 ms RACH preamble, the time available for guard time and cyclic prefix is 2 ms−0.8 ms=1.2 ms. This RACH range extension proposal attempts to reduce the receiver complexity of the RACH preamble detection.
FIG. 2 conceptually illustrates one conventional RACH receiver 200. The receiver 200 monitors signals received within the 2 ms transmission time interval of the random access channel. If the mobile unit is very close to the receiver 200, then the subframe may begin very near the beginning of the transmission time interval, as indicated by the subframe 205. However, if the mobile unit is near the edge of the coverage area of the base station, and the subframe may begin very late in the transmission time interval, as indicated by the subframe 210. A conventional preamble detection scheme may be used in this range extension scenario by shifting the starting reference time to the end of extended cyclic prefix, e.g., by shifting the Fast Fourier Transform data collection window by 0.6 ms for a 90 km coverage area, as shown in FIG. 2. The accumulated data can then be processed to search for a peak over a delay of approximately 0.6 ms.
Two partitions between cyclic prefix and guard period can be envisioned: In one case, the 1.2 ms portion of the subframe that is not allocated to the preamble could be evenly allocated to the cyclic prefix and the guard time so that the RACH coverage is extended to 90 km as shown in FIG. 2. Alternatively, the 1.2 ms portion of the subframe that is not allocated to the preamble could be unevenly distributed between the cyclic prefix length and the guard time. The uneven distribution of the allocated time to the cyclic prefix and the guard time could extend the coverage to the 100 km if the cyclic prefix length is equal to or greater than 0.667 ms. However, inter-symbol interference may occur when the cyclic prefix and guard time allocations are uneven in cases where the preamble is transmitted by a mobile unit near the cell edge. However, the signal strength received from mobile units is the edge of an extended cell, e.g., mobile units that are as much as 90 or 100 km from the base station, may be very low, which may reduce the likelihood of detecting the preamble of the random access channel message.
FIG. 3 shows a second proposed modification to a random access message 300. In this proposal, the RACH preamble 305(1-2) is repeated within each subframe 300. The energy of both RACH preambles 305(1-2) may then be accumulated for detection. The accumulated RACH preamble energy may provide the additional link performance gain to help overcome the propagation loss due to the long range. Simulation results have shown that a gain of 2.3 dB may be achieved for a false alarm probability of 10−3 in the AWGN channel. However, repeating the RACH preamble 305(1-2) reduces the time available for the cyclic prefix and the guard time. For example, if the subframe 300 includes to 0.8 ms RACH preambles 305(1-2), only 0.4 ms are left for the cyclic prefix 310 and guard time 315. The cyclic prefix length may then be set at 0.1 ms and the remaining 0.3 ms can be allocated to the guard time to extend the coverage from around 15 km to 45 km without generating inter-symbol interference. However, maintaining the cyclic prefix at 0.1 ms requires a RACH preamble detection scheme that implements testing of multiple hypotheses for users in the range larger than 15 km.
FIG. 4 conceptually illustrates one embodiment of a receiver 400 that implements multiple hypotheses testing to detect a RACH preamble over an extended range. The receiver 400 implements a set of parallel RACH preamble detection processes that each detect a disjoint range of possible RACH transmission location. For example, each of the parallel RACH preamble detection processes may compare the received signal to a reference signal such as the reference signals 1-N shown in FIG. 4. Each of the reference signals 1-N may be used to detect RACH preambles from users in different distance ranges. For example, one reference signal (and associated parallel detection process) may be used to detect users in the range of 0-15 km, another reference signal (and associated parallel detection process) may be used to detect users in the range of 15-30 kilometers, and another reference signal (and associated parallel detection process) may be used to detect users in the range of 30-45 km. However, the receiver 400 is much more complicated to implement and operate than conventional receivers, such as the RACH receiver 200 shown in FIG. 2. Moreover, the RACH detection range of the receiver 400 is limited to around 45 km.